1. Field of the Invention
The present invention relates to a punch unit for punching a hole (hereinafter referred to as xe2x80x9cpunch holexe2x80x9d) in a soft metal sheet such as an aluminum sheet and the like (hereinafter also referred to as xe2x80x9cworkpiecexe2x80x9d), wherein the punch hole assumes a circular shape or any other suitable shape, and the soft metal sheet having been punched is used in preparing a printing plate in the field of printing plate making.
2. Description of the Related Art
Heretofore, in order to punch a soft metal sheet such as an aluminum sheet and the like used in preparing a printing plate in the field of lithographic printing plate making, a punch unit of a reciprocating type has been used as is in the case where a piece of paper is punched as a workpiece.
However, in case that the soft metal sheet such as the aluminum sheet and the like is punched by using a conventional punch unit heretofore having been used in punching a piece of paper, a fragment of the soft metal sheet tends to stick to a peripheral surface of the punch when the soft metal sheet is punched. In the case where the fragment of the soft metal sheet is accumulated on the peripheral surface of the punch to fill up a radial clearance between the punch and the corresponding die, a frictional drag of the punch drastically increases to make it difficult for the punch unit to continue the punching operation of the soft metal sheet. Eventually, the punch unit fails to perform the punching operation.
Further, since the soft metal sheet tends to stick to the punch when punched, the thus formed punch hole of the soft metal sheet is poor in quality. This is a problem inherent in the conventional punch unit.
Consequently, it is an object of the present invention to solve the above problem by providing a punch unit for punching a precise high-quality punch hole in a soft metal sheet in a steady manner.
In accordance with a first aspect of the present invention, the above object of the present invention is accomplished by providing:
In a punch unit of a reciprocating type provided with a punch and a die both adapted to punch a punch hole in a soft metal sheet in a punching operation, wherein the punch is slidably guided by a punch guide and reciprocally driven in a condition in which the die cooperates with the punch in the punching operation, the improvement wherein: a cleaning means for removing a fragment of the soft metal sheet from the punch in the punching operation is provided, wherein the fragment has been stuck to the punch in the punching operation; and, removal of the fragment of the soft metal sheet from the punch is carried out by reciprocally driving the punch.
In the conventional punch unit described above, the soft metal sheet (workpiece) tends to stick to the punch. Consequently, in the punch unit of the present invention, there is provided the cleaning means. The cleaning means performs a cleaning operation in which the cleaning means aggressively removes or scrapes the fragment of the soft metal sheet from the punch when the punch is reciprocally driven in the punching operation.
Preferably, the cleaning means is constructed of a punch drive and control means. The punch drive and control means realizes a cleaning or scraping operation in which the fragment of the soft metal sheet is scraped from the punch when the punch performs its lost motion a predetermined times after a predetermined pieces of the punch holes have been formed in the soft metal sheet.
As described above, the punch is driven and controlled by the punch drive and control means to perform the lost motion at predetermined time intervals in the punching operation, so that the soft metal sheet""s fragment having stuck to the punch is scraped off the punch. For example, it is possible for the punch unit of the present invention to perform the cleaning operation of the punch in a manner such that the punch performs its lost motion three times after completion of every 10 times of punching operations of the soft metal sheet. It is also possible for the punch unit of the present invention to have the punch perform its lost motion several times a day.
Further, in the punch unit of the present invention, preferably: a gate is located in the side of the die so as to be adjacent to the punch guide; and, the cleaning means is constructed of the gate, wherein, when the punch is reciprocally driven in the punching operation, the gate permits the punch to pass through the gate and thereby scraping the fragment of the soft metal sheet off the punch to serve as the cleaning means.
In the punch unit of the present invention having the above construction, the gate forms a cutting edge for scraping the fragment of the soft metal sheet off the punch. More specifically, when the punch passes through the gate, the cutting edge of the gate scrapes the fragment of the soft metal sheet off the punch. Due to this, in the punch unit of the present invention, there is no fear that the fragment of the soft metal sheet is accumulated on the peripheral surface of the punch. Consequently, there is no fear that a frictional drag of the punch increases in the punching operation of the soft metal sheet. The gate cooperates with the punch drive and control means to remarkably improve the punch unit of the present invention in effect in the cleaning operation.
Incidentally, any other cleaning means may be realized by combining the above cleaning means with any one of a cleaning blade, a mechanical grinding unit, a gas-blower unit and the like. It is also possible to use a single cleaning means in place of combination of a plurality of cleaning means.
Further, in the punch unit of the present invention, preferably: the gate is located in the side of die so as to be adjacent to the punch guide; and, a sheet hold-down means for holding down the soft metal sheet when the punch is retracted from the soft metal sheet after completion of the punching operation is constructed of the gate. The sheet hold-down means, that is, the gate functions to prevent the soft metal sheet from being lifted together with the punch when the punch is retracted after completion of the punching operation. The gate also functions to prevent the soft metal sheet from entering the punch guide (bore). When the soft metal sheet enters the punch guide, the soft metal sheet is completely deformed. Consequently, the punch unit of the present invention is capable of preventing such deformation from occurring in the soft metal sheet.
Further, in the punch unit of the present invention, preferably: the punch guide is divided into an upper guide and a lower guide; the upper guide functions to guide the punch in its drive side; and, the lower guide functions to guide the punch in the side of the die. As a result, it is possible for the punch unit of the present invention to support opposite end portions of the punch. This effectively stabilizes the punch in its sliding motion during the punching operation, and makes it possible to reduce the length of the shaft portion of the punch, and therefore makes it possible to reduce the length of the punch guide. This leads to reduction in frictional drag of the punch, and also leads to reduction in weight of the punch unit.
Preferably, the punch unit of the present invention is further provided with a self-lubricating mechanism for applying a lubricant to the punch. The self-lubricating mechanism is preferably provided in a die block of the punch unit, which may reduce a necessary power for driving the punch and improve each of the punch and the die of the punch unit in service life. Further, the self-lubricating mechanism may be constructed of a felt member in an easy manner. The felt member has been impregnated with a suitable lubricant such as a lubricating oil and the like, and is preferably provided between the upper guide and the lower guide in a condition in which the felt member is brought into contact with the shaft portion of the punch.
Further, preferably, an opening end portion of the punch guide is disposed in the side of a workpiece passage, and punch guide does not interfere (contact) with punch""s interference area brought into contact with said soft metal sheet.
In other words, the above arrangement of the punch unit of the present invention prevents the punch having stuck to the soft metal sheet from interfering with the punch guide. Consequently, even when the fragment of the soft metal sheet sticks to the punch in the punching operation, in the punch unit of the present invention, there is no fear that a frictional drag of the punch increases.
Further, in the punch unit of the present invention, the punch is preferably coated with a suitable anti-adhesion agent. This makes it possible to further reduce the frictional drag of the punch, and also makes it possible to prevent the soft metal sheet from sticking to the punch in the punching operation.
Further, in the punch unit of the present invention, preferably, the grooves for reducing a peripheral sliding contact surface area of punch and punch guide is formed at least one of the punch and punch guide. The grooves may be constructed of an annular groove formed in an outer peripheral surface of the punch. It is also possible for the grooves to assumes a plurality of longitudinal grooves which are formed in the outer peripheral surface of the punch and/or in an inner peripheral surface of the punch guide.
Still further, preferably, a thickness of the die is set at a value of from 0.5 to 1.2 mm to reduce the amount of its interference area with respect to the punch in the punching operation, which may effectively reduce the frictional drag of the punch slidably driven in the punch guide during the punching operation. In general, the more the thickness of the die is reduced, the more the fragment of the soft metal sheet having been punched by the punch becomes hard to stick to the punch. On the other hand, the more the thickness of the die is reduced, the more the cutting edge portion of the die is also reduced in mechanical strength. Experiments show that: the die is poor in mechanical strength when used together with its cutting edge portion having a thickness of less than or equal to 0.5 mm; and, the fragment of the soft metal sheet having been punched by the punch tends to stick to the punch when the cutting edge portion of the die has a thickness of more than or equal to 1.2 mm. In order to improve the cutting edge portion of the die in mechanical strength, it is preferable to form the cutting edge portion or bore of the die into a stepped shape. More specifically, it is preferable to provide both a land portion and an undercut angle portion in the bore of the die.
In case that the punch hole of the soft metal sheet does not assume a notch-like shape but assumes a circular shape, a square shape or any other suitable closed-edge shape in opening, it is preferable for the punch to assume a stepped shape constructed of: a shaft portion; and, a cutting edge portion which is differs from the shaft portion in diameter. Such stepped shape enables the punch to be precisely aligned in center line with the punch guide, and also enables the punch to set a radial clearance between the punch and the die at an appropriate value.
In case that the punch unit of the present invention for punching the notch-like shape in the soft metal sheet is characterized in that a deviation prevention means for preventing the punch from deviating from its correct center line in the notch punching operation is constructed of: a cutting edge portion of the punch, wherein the cutting edge portion covers the entire cut portion of the notch, wherein a radial clearance between the punch and a bore of the die has a predetermined value; and, the shaft portion of the punch, which shaft portion is precisely and slidably guided by the punch guide in the notch punching operation.
The punch for punching a notch in the edge portion of the soft metal sheet differs in shape from an ordinary punch for punching an ordinary punch hole or closed-edge hole. Due to this, in case that the notch punching operation is performed, it is necessary to form the ordinary punch into a stepped shape. However, such shaping operation of the punch is very expensive. When the ordinary punch used in punching the punch hole is used to punch a notch in the edge portion of the soft metal sheet without shaping the punch into the stepped shape, the punch tends to deviate from its correct center line, which makes it difficult to keep the radial clearance between the punch and the die at the appropriate value. As a result, the fragment of the soft metal sheet having been punched by the punch tends to stick to the punch. In order to solve this problem, in the punch unit of the present invention, the punch""s shaft portion (i.e., guide portion) is not brought into contact with the notch thus formed in the edge portion of the soft metal sheet, and is slidably supported by the die during the notch punching operation to prevent the punch from deviating from its correct center line, which enables the punch to be precisely guided during the notch punching operation. Due to the above arrangement, it is possible for the punch unit of the present invention to keep the radial clearance between the punch and the die at an appropriate value even in the notch punching operation. This enables the punch used in the punch unit of the present invention to punch a notch in the edge portion of the soft metal sheet in a steady manner, and prevents the fragment of the soft metal sheet thus punched from sticking to the punch.
The punch unit of the present invention is most effectively used in the case where the soft metal sheet is an aluminum sheet or an aluminum-alloy sheet. However, it is also possible to use the punch unit of the present invention in punching any other soft metal sheet such as a copper sheet, a copper-alloy sheet, a lead-alloy sheet and the like.
Also, The punch unit of the present invention is effectively the punching in the soft metal sheet is used in the field of printing plate making.
As described above, in the punch unit of the present invention having the above construction, it is possible to punch a high quality punch hole in the soft metal sheet, and also possible to improve both the punch and the die in service life. This makes it possible to improve the punch unit of the present invention in accuracy and in efficiency in punching an aluminum sheet and like workpiece when a printing plate used in the lithographic printing operation is prepared.